Swedish Intervention

The Swedish Intervention was the penultimate stage of the Thirty Years' War, during which Sweden (led by King Gustavus Adolphus) assumed the leadership of the Protestant cause and turned the tide of the war against the Catholic League. Sweden emerged as a great power as a result of its involvement in the war, acquiring Pomerania and becoming the new dominant power in the Baltics.

Background
Sweden had first become involved in the Thirty Years' War in 1628, when it garrisoned Stralsund during the siege of the city by Albrecht von Wallenstein's German Catholic army and helped to repel the Imperial forces. In the aftermath of Denmark's Treaty of Lubeck with the Holy Roman Empire in 1629, King Gustavus Adolphus feared that the Holy Roman Empire would ally with Poland-Lithuania and help King Sigismund III with pressing his claim to the throne of Sweden. In the summer of 1630, the Diet of Regensburg was held in the Holy Roman Empire in an attempt to calm tensions rising from the Edict of Restitution (which confiscated any Catholic-majority lands ruled by Protestnat rulers), but Saxony and Brandenburg refused to appear. Emperor Ferdinand II was forced to downsize the Imperial army in order to decrease its high upkeep costs, and the electors who hated Wallenstein forced Ferdinand to fire Wallenstein and reinstate Count Tilly as commander of the imperial army. This alienated the Protestant princes further, encouraging Sweden to join the conflict and take advantage of the Holy Roman Empire's weakness. In June 1630, protesting against Imperial aggression against the Stralsund garrison and disrespect shown towards Swedish diplomats, Gustavus Adolphus declared war on Ferdinand.

War
On 26 June 1630, Gustavus Adolphus and 14,000 Swedish troops landed on the Pomeranian island of Usedom and marched on Stettin, where he forced the childless Bogislaw XIV of Pomerania to allow for Sweden to annex the new region. Throughout the rest of 1630, repeated and unsuccessful attempts to punch through the Imperial cordon under Torquato Conti failed, and Ferdinand was instilled with a false sense of confidence. Shortly after, Cardinal Richelieu of France began to fund the Swedes in order to keep their Habsburg rivals occupied, obliging Sweden to maintain an army of at least 36,000 troops while paying Sweden 400,000 thalers a year to subsidize its army. Saxony and Brandenburg initially viewed the Swedes as an unwelcome foreign invader, and, on 12 April 1631, they issued the Leipzig Manifesto and formed a defensive alliance against both Sweden and the Holy Roman Empire, raising 40,000 troops. In Pomerania, Gustavus Adolphus' swelling army reached 30,000 troops, and Gustavus Adolphus advanced south into Brandenburg in April 1631 and captured and sacked Kustrin and Frankfurt an der Oder, which were held by Imperial garrisons. Meanwhile, Count Tilly's army entered Magdeburg on 20 May 1631 and burned and sacked the city, with the underpaid German mercenaries massacring the 25,000-strong population down to 449 people. The massacre shocked and outraged Protestants, and Gustavus Adolphus forced Elector George William of Brandenburg to ally with him against the Imperial forces. Meanwhile, John George I of Saxony joined the Swedish cause with his 18,000-strong army, and Gustavus Adolphus sought a decisive victory over the Imperial forces to inspire more Protestant princes to join him.

The first great battle was the First Battle of Breitenfeld in September 1631, during which Gustavus utterly annihilated Count Tilly's Imperial army, forcing the Imperials to rebuild their army from scratch as Gustavus' ranks were swelled by newer Protestant allies. Gustavus was nicknamed "the Lion of the North", and he now had to defend the Baltic coast beachhead from attack, prevent the wavering Saxons from making peace, and push his advantage. Tilly escaped west through Westphalia and then south into Franconia to join his reinforcements, proving to be a thorn in Gustavus' side. Gustavus decided to thrust southwest through Thuringia and into Catholic country before winter could arrive, hoping to enlist the help of Protestant allies along the way. Opposition was unexpectedly weak, and Erfurt fell on 2 October, while Wurzburg fell on 15 October; its garrison pleaded for mercy while the Protestants responded with "Magdeburg's Quarter" (massacring them). Gustavus crossed the Rhine at Oppenheim and took Mainz on 23 December, followed by Heidelberg and the rest of the lower Palatinate in the following weeks. Meanwhile, Prague was recaptured for the Protestant cause by the Saxons.

At the beginning of 1632, Gustavus Adolphus commanded 16,000 Swedish troops, while Marshal Horn led 10,000 troops to subdue the Catholics of Franconia. Tilly attempted to avoid battle as his soldiers detested their crowding in Bavaria. Emboldened Protestants and Catholics began to raise smaller armies across Germany and fight against each other, while the Swedes sent out smaller armies to ravage the countryside. Pappenheim conducted brilliant campaigns in the north to relieve trapped Imperial contingents in Magdeburg and Stade while destroying larger Protestant armies. Horn attacked Tilly at Bamberg, but his besieging force lost a third of its strength on 9 March 1632. Worried that the defeat might make the wavering Protestant Germans desert him, Gustavus captured Nuremberg. Gustavus then focused on capturing Munich in southern Bavaria, and he chose to move towards Rain to cross the Lech River, intending on fighting Tilly's army. The ensuing Battle of Rain on 5 April 1632 saw Tilly die in battle and Maximilian I of Bavaria, the new commander of the Catholic army, order a retreat rather than face destruction. The Emperor failed to call on Spain, Lorraine, and Spain for help, so he called Wallenstein back into his service as supreme commander of Imperial forces. He immediately rebuilt shattered Imperial regiments and stockpiled resources, and, by the end of May, he had recaptured Prague and evicted them from Bohemia by late spring. Wallenstein and his 32,000-strong army marched west to face King Gustavus, who was now facing mounting resistance from Catholic cities. Gustavus marched back north to Nuremberg, where he fought Wallenstein in the Battle of the Alte Veste, resulting in a draw. Meanwhile, to the north, the Battle of Steinau resulted in a Saxon victory, but Heinrich Holk and a 10,000-strong Imperial army devastated Saxony in revenge.

When the Battle of the Alte Veste ended, Gustavus and his army marched south into Swabia, believing that Wallenstein's army - which had recently been immobilized by sickness - no longer posed a major threat. Wallenstein quickly recovered and marched north to crush Saxony, and Gustavus - fearing that his Saxon allies would be crushed and that his Baltic communication and supply routes were at risk - rushed north with his army. Wallenstein and his army then met the Swedes in the Battle of Lutzen. The ensuing battle saw both sides suffer heavy losses, and Gustavus Adolphus was killed in battle, another turning point in the war. The battered and warring parties were forced to seek foreign support, with Sweden hoping for French aid and the Emperor hoping for Spanish assistance. Gustavus' heir Christina of Sweden was only six, so the Swedish chancellor Axel Oxtenstierna formed the Heilbronn League with the Swabia, Franconia, and Rhine regions of Germany and renewed the alliance with France. 1633-1634 had also seen the explusion of Spain's troops from the Palatinate and Lorraine, and the Spanish Road was blocked by Franco-Swedish troops. Wallenstein was assassinated by mercenary officers shortly after, and Ferdinand had his son Ferdinand of Hungary assume command of his army.

In 1634, Saxon forces besieged Prague as Heilbronn League forces thrust south and invaded Bavaria, while the Imperials recaptured Regensburg and Donauworth; Spanish and Imperial forces also defeated the Swedes at the First Battle of Nordlingen. In May 1635, the Protestant princes agreed to sign the Peace of Prague with the Emperor, who amnestied his former enemies and revoked the Edict of Restitution. He combined the Catholic and Protestant armies into one army, bringing an end to the religious conflict.

On 19 May 1635, however, France declared war on Spain as a limited and indirect intervention. The veteran Spanish armies repelled a Franco-Dutch campaign at Namur and a French invasion of the Rhine. However, Cardinal Richelieu created several proxy wars against Spain, including convincing Savoy to declare war on Spain. On 20 May 1636, France and Sweden signed the Treaty of Wismar to unite their armies against the Habsburgs; the Swedes doggedly held onto their bridgehead in Germany rather than return home, and Baner advanced down the Elbe with 17,000 troops before badly defeating an Imperial-Saxon army in the Battle of Wittstock in October 1636. To the west, the veteran Spanish Army of Flanders under Cardenal-Infante Fernando de Espana invaded France itself and nearly captured Paris before it was repelled back into Spanish territory. The back-and-forth campaigns of 1636 set the pattern for the remainder of the war.

In February 1637, Emperor Ferdinand II died at the age of 58, and Ferdinand of Hungary became "Ferdinand III". Meanwhile, France gradually intensified its campaigns under Bernhard of Saxe-Weimar, who invaded Franche-Comte and Alsace, failing to maintain any conquests. The Swedish-Imperial conflict continued, with Baner's troops escaping from an Imperial encirclement at Torgau. The Swedish garrisons in Pomerania were unpaid and had decaying fortifications, and, by the end of 1637, Sweden was only left with Stettin, Stralsund, Wismar, and some other coastal cities, as the Imperial forces had picked off many of the other garrisons. In 1638, Saxe-Weimar won the Battle of Rheinfelden and proceeded to besiege Breisach after starving it out. Baner then received reinforcements, broke the Imperial cordon, and invaded Saxony, taking Zwickau and Chemnitz and taking Freiberg after defeating the Saxons. He went on to besiege Prague until a 30,000-strong Imperial relief army chased them away, and the Swedes burned the Bohemian countryside while retreating to Erfurt. The French also attacked the Spanish in the Pyrenees, and, in 1640, Catalonia and Portugal rebelled against Spain, spreading their forces thin. Swedish troops under Lennart Torstenson retreated to Breitenfeld, where he won the Second Battle of Breitenfeld in 1642. In May 1642, the Spanish won the Battle of Honnecourt, clearing the way to Paris, and, on 4 December 1642, Richelieu died, being replaced by Cardinal Mazarin. In 1643, the Spanish had all the initiative along the Rhine as King Louis XIII of France lay on his deathbed. Rather than focus on the Netherlands, the Spanish general Francisco de Melo decided to continue the invasion of France, and, on 10 May 1643, his forces crossed the French border and planned to take the small fortress of Rocroi. The 22-year-old Louis II de Bourbon, Prince de Conde met the Spanish in the Battle of Rocroi on 19 May 1643, and the French utilized superior firepower to slaughter the Spanish tercios and decisively defeat them.